Jun

10

Startup Enphase Energy of Petaluma, CA, is now making its first micro-inverters. The small inverters can be bolted to the racking under each of an installation’s solar panel to convert DC power into AC for each panel individually. The company claims that the devices will increase a PV system’s efficiency by 5 to 25 percent and decrease the cost of solar power.

There’s much more to solar power than black glassy panels glistening on rooftops. Perhaps more important now that installations and real world testing is well underway and understood is the inverter performance that convert DC power created by the solar panels into grid-ready AC power.

Currently all the panels in a rooftop photovoltaic system are connected to one large inverter mounted on the side of a house from which the AC power is off loaded to the house or grid. This is being done as solar panels are wired together in series, and their combined high-voltage DC power is fed to the inverter. From that current flow the inverter’s logic circuit optimizes the total current and voltage levels. But if one panel’s current drops, it becomes the limit of the overall output of the system.

Leesa Lee, director of marketing at Enphase points out the problem, “Something as simple as a leaf blowing over a module, or dust or debris or shade on one module, will affect the entire array of all those modules that are connected in series.” Think bird poop and all the other things falling out of the sky as major problems, but mostly canceling the equality of each panel, that forces production to the least efficient module. It’s a bigger problem than many realize.

Enphase Micro Inverter Points. Click image for a larger view.

But Enphase’s micro-inverters individually optimize the voltage-current levels at each panel. That uses the most power from each panel and then adds the panels together, increasing the system’s efficiency. “Any problem on a module is limited to that module alone,” Lee says. In addition, the equipment cost for micro-inverters is about 15 percent less than the cost for a traditional system, she says, because expensive DC components, such as signal combiners and disconnects, can be replaced with off-the-shelf AC parts.

The problem has been known for decades so the concept of small inverters has been around for more than a decade, but there have been technical challenges to making practical devices. Enphase’s Senior Director for Systems, Mary Dargatz says, “One of the biggest stumbling blocks to micro-inverter technologies in the past has been conversion efficiency.” So, Enphase has converted many analog parts in the circuits to digital to make the inverter smaller without sacrificing efficiency. The conversion efficiency of an individual micro-inverter is 95.5 percent, on par with efficiencies of traditional large inverters, which range from 94 to 96 percent.

Going partway in an attempt to address a broader voltage range, National Semiconductor is making a power-optimizing module for individual panels. The device only has the logic circuit for optimizing current and voltage levels–it doesn’t do the DC-to-AC power conversion. What it offers in conversion efficiency looks to be meant for existing installations.

Enphase uses its AC output and ease of connection to offer another service to backup the sale. The full kit would allow a consumer to send data in for analysis and receive reports via the Internet. Beyond that, the potential exists for rationing power, if the situation allows, to divide one’s output say for use in the home and for sale.

It all makes for a much more practical implementation of solar arrays with photovoltaic collector panels. A drop in panel costs, now a drop in inverter cost and a simpler installation should help get home and small commercial arrays more deeply down into the economy where more people can afford the investment. That more mass market, which should reduce prices as well.

Which brings us to what might be the most important advantage of all. With the Enphase micro-inverter one can start small and add modules or panels as the budget (or incentives) allow. Now that’s a path to help build more market, too. Growth looks good for photovoltaic.

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